ATRにおけるGd入り燃料利用特性評価

Coolant void reactivity estimation of ATR by using gadolinia fuel rod

福村 信男*; 川太 徳夫*; 若林 利男*

Fukumura, Nobuo*; not registered; not registered

(目的)ATR炉心にガドリニア入り燃料を用いた場合の冷却材ボイド反応度の低減効果を定量的に評価し、その現象解明を行う。(方法)重水臨界実験により精度評価された核特性解析コードシステムWIMS-CITATIONを用い、ATR炉心でのガドリニア入り燃料の最適効果を冷却材ボイド反応度の低減量より評価する。(結果)ガドリニア濃度、ガドリニア入り燃料棒本数およびガドリニア入り燃料棒の集合体内配置をパラメータとして冷却材ボイド反応度の低減量を最大にする最適パラメータを決定した。この結果36本燃料集合体の内・中間層にガドリニア濃度5W/Oのガドリニア入り燃料棒本数各3本を配置すると燃焼末期(24GWD/T)でも冷却材ボイド反応度を負の値にできることが判明した。(結論)冷却材ボイド反応度が正になりやすいATR型炉心でも、バーナブルポイズンとして有効なガドリニア入り燃料棒を効果的に燃料クラスタ内に配列することにより、燃焼期間を通じて冷却材ボイド反応度を常に負の値にする解が得られた。

It is a difficult problem to shift coolant void reactivity of ATR to more negative. However it has been clarified by critical experiments with using DCA that use of the fuel pins containing gadolinia is very effective for decreasing coolant void reactivity. Then the analytical study has been done in order to find the solution which has the negative coolant void reactivity of ATR. The lattice is composed of the 36-rod fuel cluster and the lattice pitch is 24.2 cm square. The average enrichment through the MOX fuel cluster is 2.53 or 2.88 w/o Pufis.. This lattice is almost the same one as the demonstration power plant of ATR which is now under designing. The calculation code which is used is WIMS ATR - CITATION code system. The calculational input parameter is as follows; (1)Position of gadolinia poisoned fuel pin. (2)Number of gadolinia poisoned fuel pin. (3)Concentration of gadolinia contained in the fuel pin. (4)Concentration of B in the DO moderator. (5)0, 20, 40, 60, 80 and 100% coolant void. The burnup calculation is up to 20 GWD/T. This is the core average value. The output of calculation is as follows: (1)Coolant void reactivity. (2)Reactivity coefficient of coolant void. (3)Local power peaking. (4)Decay curve of gadolinia concentration through burn up. According to the present study, the following understanding is clarified: (1)The best solution which has the negative coolant void reactivity value through burn up is the use of the 36-rod MOX fuel cluster which has the gadolinia poisoned fuel pin both in the inner and in the middle array of the cluster. (2)The number of the gadolinia poisoned fuel pin is 3 both in the inner and in the middle array. (3)Concentration of the gadolinia is 5 w/o. It is explained that the above effect is due to decreasigg the thermal neutron shielding of HO coolant by the strong neutron absorber gadolinia.